1. Field of the Invention
The present invention relates to a position detection apparatus which detects the position of a movable member relative to a fixed member.
2. Description of the Related Art
Conventionally, as an apparatus for measuring the moving distance of an object, there is known, an absolute encoder which can measure an absolute position as well as an incremental encoder which measures a relative moving distance.
Japanese Patent Application Laid-Open No. H08-304113 discloses a Vernier-type absolute encoder. This encoder is configured to have a main track constituted by lattice patterns with different pitches and at least one sub-track. The encoder performs absolutization in one circulation interval based on the slight shifts between detection signals originating from the pitch differences between the respective tracks.
In addition, Japanese Patent Application Laid-Open No. H05-45151 discloses an invention concerning the timing of absolutization in a Vernier-type absolute encoder. This encoder is configured to have tracks formed with fine coarse pitches. When performing absolutization from these tracks, the encoder performs the absolutization at the timing when the velocity drops to the velocity at which no false detection occurs in incremental measurement with the fine pitch.
However, the absolute encoder disclosed in Japanese Patent Application Laid-Open No. H08-304113 has the following problem. In general, when calculating an absolute position from signals originating from a plurality of tracks, it is necessary to ensure synchronization between the signals originating from the respective tracks of the absolute encoder. This is because of the premise that, signals originating from a plurality of tracks are those obtained when the position of a measurement target object remains the same or within a given allowable range. Assume that the encoder time-divisionally detects signals originating from a plurality of tracks. In this case, if a measurement target object is in a state of movement, the respective signals originating from the plurality of tracks become signals at the different positions of the measurement target object. This breaks the above premise, leading to a problem that the encoder cannot calculate a correct absolute position. In addition, there is also available a method of ensuring synchronization between a plurality of track signals by using a sample/hold circuit and the like. This however raises a problem that the addition of a circuit, will increase the cost and it takes more time to hold signals.
In addition, the absolute encoder disclosed in Japanese Patent Application Laid-Open No. H05-45151 has the following problem. A false detection, in Japanese Patent Application Laid-Open No. H05-45151 is not based on the premise that the encoder time-divisionally detects signals originating from a plurality of tracks, and hence only a false detection occurs in incremental measurement with the fine pitch. In order to calculate a correct, absolution position, however, it is important that each of signals originating from a plurality of tracks is the one at the position of a measurement target object within a given allowable range. Even if, therefore, there is no false detection in incremental measurement with the fine pitch, it cannot be said that the encoder ensures the synchronization between signals originating from a plurality of tracks, and it is not possible to calculate a correct absolute position. That is, if the encoder determines the calculation of an absolute position based on a false detection in incremental measurement with the fine pitch, the encoder calculates a wrong absolute position.